Ultrasound-Assisted Extraction of Taro Leaf Antioxidants Using Natural Deep Eutectic Solvents: An Eco-Friendly Strategy for the Valorization of Crop Residues

Abstract

Colocasia esculenta L. leaves are considered a by-product of taro cultivation and are discarded as environmental waste, despite their valuable phenolic composition. Their valorization to obtain value-added substances for medicinal, food, and cosmetic applications is the aim of the current work. An ultrasound-assisted extraction was developed for the environmentally friendly and sustainable isolation of taro leaf antioxidants using natural deep eutectic solvents (NaDESs). Among the utilized solvents, the NaDES based on betaine and ethylene glycol provided the best extraction efficiencies in terms of polyphenolic content and antioxidant activity. Multi-response optimization suggested a solvent-to-solid ratio of 10 mL g^-1, a processing time of 60 min, an extraction temperature of 60 °C, and a water content of 33.8% (w/w) as optimal extraction parameters. Leaf extract obtained under these optimum operational parameters demonstrated a strong radical scavenging activity against 2,2-diphenyl-1-picrylhydrazyl (65.80 ± 0.87%), a high ferric reducing antioxidant power (126.62 ± 1.92 μmol TE g^-1 sample), and significant protection against oxidative stress-induced DNA damage. The chromatographic characterization of the optimum extract revealed its richness in flavonoids (flavones and flavonols). The outcomes of the present study suggest that the proposed method could serve as a highly efficient and green alternative for the recovery of polyphenols from agricultural wastes.

Keywords: Colocasia esculenta L.; antioxidants; eco-friendly extraction; green chemistry; natural deep eutectic solvents (NaDESs); polyphenols; response surface methodology (RSM); taro leaves; ultrasound-assisted extraction (UAE); waste valorization.

Figure 1

Fourier-transform infrared spectra of Bet, EtGl, and the prepared NaDES. The blue spectrum represents the hydrogen bond acceptor (HBA) (Bet), the red spectrum represents the hydrogen bond donor (HBD) (EtGl) and the grey spectrum represents the NaDES.

Figure 2

The effect of NaDES composition on (A) the total content of extracted polyphenolic components (TPC, TFC, THA, TF) and (B) the antioxidant activity (% inhibition of DPPH radical, FRAP) of the extracts. Different lowercase letters for each colored bar indicate significant differences (p < 0.05) according to Duncan’s multiple range test.

Figure 3

Principal component analysis (PCA) biplot demonstrating the distribution of extracts obtained using the prepared NaDESs and conventional solvent (40%, v/v, ethanol (EtOH)).

Figure 4

Response surface plots demonstrating the interactive effects of solvent-to-solid ratio, processing time, extraction temperature, and water content on TPC.

Figure 5

Response surface plots demonstrating the interactive effects of solvent-to-solid ratio, processing time, extraction temperature, and water content on DPPH.

Figure 6

Protection offered by taro extracts against H₂O₂-induced DNA damage. Jurkat cells (150,000 cells per 100 μL) were preincubated for 30 min with the indicated concentrations of the taro extract (red bars) before being exposed for 15 min to continuously generated H₂O₂ (10 μM H₂O₂ per min) by the action of the glucose oxidase (G.O.) enzyme (green bars). DNA single-strand breaks were evaluated by the single-cell gel electrophoresis method (or comet assay) and expressed in arbitrary units as described in Materials and Methods. Each point represents the mean of three different experiments performed in duplicate. Different lowercase letters for each colored bar indicate significant differences (p < 0.05) according to Duncan’s multiple range test.